Photogrammetry is the practice of determining geometric properties of objects from photographic images. Photogrammetry is frequently used in the modelling of 3-dimensional (3D) objects from 2-dimensional (2D) photographs. By taking multiple photographs of an object from different angles and/or different positions, “lines of sight” from the camera to different features or points on the object can be determined. Mathematical intersections of these lines of sight are subsequently used to determine, in a process known as triangulation, the 3D coordinates of those features of the object.
The relative success of photogrammetric measurement typically depends on a number of factors, including the quality and resolution of the camera taking the photos, the size of the object being photographed and subsequently measured, the number of photos taken of the object, and how the photos are oriented relative to each other and to the object. Moreover, successful measurement also depends on elements such as the visibility of key features of the object, as well as how well the features are targeted. Precise targeting enhances the accuracy of photogrammetric measurements.
In at least some cases, targeting is performed using retroreflective targets. Generally, these retroreflective targets are small pieces of highly reflective material that reflect light back to its source with minimal scattering. In some embodiments, the retroreflective material is as much as 100 to 1000 times more efficient than a white (non-retroreflective) target. Accordingly, only a low-power flash is necessary to illuminate the targets. The targets in the photograph are very bright and, thus, are easy to locate and measure. At least some known targets are manufactured with an adhesive backing layer so that the targets are adhered or placed upon key features of the object being photographed. Targets are placed individually on the features, and measurements are taken by hand or computed between adjacent targets. However, this leaves room for computer and/or human error. Accordingly, at least some retroreflective targets are manufactured on a band, in which the targets are attached to a matte black tape and are equidistant from each other, further enhancing the precision of the targets and, thus, the accuracy of the photogrammetric measurements. However, at least some known systems provide a band of retroreflective targets that may induce poor performance due to interference from the raised sides of the target.
A need exists, therefore, for bands including retroreflective targets that overcome the failures of the prior art and lead to improved performance through, for example, improved tolerance of the targets.
Like reference symbols in the various drawings indicate like elements.